The electronic instrumentalities forming the basis for equipment such as mobile phones, computers, and servers are generally fabricated by electrically interconnecting electronic components such as integrated circuits, resistors and capacitors with printed circuit boards. The electrical input and output of the components to a second component, for example, a printed circuit board is generally accomplished through a conducting region typically denominated a pad. Such pads generally have dimensions in the range 100 μm2 to 1 mm2 and are electrically connected to the circuitry of the component such as a printed circuit board through conducting pathways denominated runners. (A runner typically has a width in the range 5 μm to 100 μm and a length, depending on the device configuration, often ranging between a few micrometers and a few centimeters.) For most applications, both the pad and runner, on, for example, the printed circuit board, are metallic and have upper surfaces that include copper, nickel, gold, silver, palladium, or aluminum. (Upper in the context of this disclosure is a surface closest to the point of electrical connection to another component.)
Two components, e.g. an integrated circuit and printed circuit board, are electrically connected through the expedient of solder ball connections, or as they are termed in the trade, solder bump or solder ball connections. Typically, the pads of the two components to be electrically interconnected are, at least in part, configured to be geometrically aligned when the components are positioned for connection. The electrical connection itself in many applications such as flip chip solder connection is made by first forming a solder bump on one (e.g. the integrated circuit pad) of each pair of pads to be electrically connected. (The solder bump is typically approximately spherical in cross section but the shape is not critical.) In manufacture, a solder mask composed generally of an electrically insulating material is configured over the pad region of, for example, the printed circuit board with openings in the mask to allow connection to the underlying pads. (See Microelectronic Packaging Handbook, R. Tummala, et. al., Chapman and Hall, New York, N.Y. 1997, for a description of typical solder mask materials and methods for configuring such masks.) The components are then electrically connected by a process (generally denominated reflow) in which the pads to be connected are aligned and the solder bumps are brought to a temperature exceeding their melting point as described by Tummala supra. This heating process causes the formation of intermetallic compounds at the solder bump/pad interface that contribute to the adhesion of the connected structure. For example, for a copper pad and a lead-free tin/silver/copper solder composition typical intermetallic compounds such as Cu6Sn5 and Cu3Sn respectively are formed at the interface. Similar intermetallics are formed when tin/silver/copper solder balls are attached to the copper pads on a circuit board during reflow.
For reasons that are not fully understood, solder bumps (or solder balls) at times fail when subjected to stress engendered by typical occurrences such as the dropping of an instrumentality or a component, or through the mechanical forces endured during transportation and handling of such entities. There are two predominant failure modes for the solder bump connection. Either the solder bump itself shears into two portions, or the bump fractures (brittle failure) near or at the intermetallic region present at the interface between the pad and the solder bump. Clearly the failure of the electrical connection, especially after the instrumentality has been assembled, is costly and quite undesirable.
In an attempt to predict the failure of components and remove such components from the manufacturing stream, a variety of tests have been developed. In one approach, a probe, 2 in FIG. 1, is traversed in direction 5 against solder bump 7 present on pad 9 with surrounding solder mask 6. The speed of the probe traversal has a significant effect on the ultimate results. Generally, faster probe speeds, i.e. speeds in excess of 1 mm/sec, lead to brittle failure i.e. failure at the intermetallic compound present at the interface from solder bump formation and/or reflow. Slower speeds, i.e. speeds less than 0.5 mm/sec, typically lead to ductile failure—a sheering of the solder bump bulk. (Speeds between 0.5 mm/sec and 1 mm/sec lead to a mixed failure mode.) Similarly, in a second approach to testing, as shown in FIG. 2, the solder bump 7 is grasped by probe 20 and stress is applied in direction 25. Again, the speed of the probe movement dictates whether a brittle or ductile failure occurs.
The failure of solder bumps has become an even more pressing problem with the ever increasing desire to eliminate lead from solder bump compositions. Lead-containing solders are more prone to undergo ductile failure than lead-free solder alloys. In contrast, lead-free solders i.e. solders having a lead content less than 300 ppm lead, have an increased tendency to suffer brittle rather than ductile failure. Unfortunately, a composition that predominantly fails by a ductile mechanism is much preferred because such failure typically occurs less frequently and occurs only after substantially longer periods of stress. Thus, the trend towards lead-free solders has made the identification of components that are likely to fail a more pressing concern. Despite the ever-increasing importance of such identification, available testing approaches have not yielded the desired level of predictability.